By : spamizbad

I currently have a small office that depends on faxing. I'd ditch fax if I could, but my clients are funny in their ways.

Anyway, I'm using Asterisk 1.6.2 with FreePBX 2.0 and Broadvox is my SIP provider. My system is set up to receive faxes on the Asterisk PBX and email them to a mailing list of employees (this is configured through FreePBX). I also have a Grandstream GXW-4004 to provide FXS ports for the fax machine and analog phones that aren't important enough to replace with expensive VOIP handsets.

Note: I never got the analog fax machine to send and receive. Depending on how I set the fax detect on the Grandstream, I could do either-or, but not both. So my PBX receives faxes and my machine sends them. This is likely a PEBKAC matter, but I can't really be bothered to put forth the time to work this final gremlin out of my system.

My advice to anyone doing faxing over VoIP is the following:

1) If possible, don't.
2) Make sure your SIP provider supports VOIP faxing. Some don't with good reason: it's a bitch to support.
3) Make sure your SIP provider support G.711 with T.38.
4) Make sure your ISP does not strip QoS headers (In this case DiffServ EF)from IP packets. Your ISP will ignore them, obvious, but you definitely don't want them stripped or overwritten.
5) Make sure your VOIP packets have a DiffServ EF value in their headers.
6) Make sure any router(s) and switch(es) are configured properly to give top priority to your VOIP traffic. Failure to do so will likely result in jitter on your calls and you'll be unable to successfully send/receive faxes.
7a) If possible, don't run your primary PBX in a virtualized enivornment. Competition for system resources may create unwanted latency on your Asterisk VM resulting in jitter and failed faxes.
7b)With that said, I would definitely keep a cold or hot "spare" PBX running in a VM to spin up in the event of an outage.

Good luck!